Add wasm32 implementation for libc

This adds a wasm32 implementation for `free`, `malloc` and `realloc`.
Originally written by @rodrigorc in
rust-lang/libc#1092 and adapted.

NOTE: This is only enabled for `wasm32-unknown-unknown` ([`target_arch =
"wasm32"`](https://doc.rust-lang.org/reference/conditional-compilation.html#target_arch), [`target_os =
"unknown"`](https://doc.rust-lang.org/reference/conditional-compilation.html#target_os)), because other wasm targets (like `wasm32-unknown-emscripten`) may provide a `libc` implementation.

Author: nmattia

src/ffi.rs

@@ -8,10 +8,17 @@ use std::slice;
 use std::fmt;
 use std::os::raw::*;
 use std::ffi::CStr;
-use std::path::*;
 use std::io;
 
 use crate::rustimpl;
+#[cfg(all(target_arch = "wasm32", target_os = "unknown"))]
+mod libc;
+
+#[cfg(all(target_arch = "wasm32", target_os = "unknown"))]
+use std::path::{PathBuf};
+
+#[cfg(not(all(target_arch = "wasm32", target_os = "unknown")))]
+use std::path::{Path};
 
 macro_rules! lode_error {
     ($e:expr) => {

src/ffi/libc.rs

@@ -0,0 +1,36 @@
+pub type size_t = usize;
+
+const MALLOC_HEADER : isize = 8;
+const MALLOC_ALIGN : usize = 8;
+
+use super::c_void;
+use std::alloc::{self, Layout};
+use std::ptr;
+
+pub unsafe fn malloc(size: size_t) -> *mut c_void {
+    let lay = Layout::from_size_align_unchecked(MALLOC_HEADER as usize + size, MALLOC_ALIGN);
+    let p = alloc::alloc(lay);
+    if p.is_null() {
+        return ptr::null_mut();
+    }
+    *(p as *mut size_t) = size;
+    p.offset(MALLOC_HEADER) as *mut c_void
+}
+pub unsafe fn free(p: *mut c_void) {
+    let p = p.offset(-MALLOC_HEADER) as *mut u8;
+    let size = *(p as *mut size_t);
+    let lay = Layout::from_size_align_unchecked(MALLOC_HEADER as usize + size, MALLOC_ALIGN);
+    alloc::dealloc(p, lay);
+}
+pub unsafe fn realloc(p: *mut c_void, _size: size_t) -> *mut c_void {
+    let p = p.offset(-MALLOC_HEADER) as *mut u8;
+    let size = *(p as *mut size_t);
+    let lay = Layout::from_size_align_unchecked(MALLOC_HEADER as usize + size, MALLOC_ALIGN);
+    let p = alloc::realloc(p, lay, size);
+    if p.is_null() {
+        return ptr::null_mut();
+    }
+    *(p as *mut size_t) = size;
+    p.offset(MALLOC_HEADER) as *mut c_void
+}
+